Engine Knock Under Load

Discussion in 'Airheads' started by Boxer Metal, Apr 13, 2013.

  1. Boxer Metal

    Boxer Metal Mad Scientist

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    So I have a motor that once it is warmed up there is an engine knock only under load and it goes away at higher RPM's No noise when cold or at revving the motor at idle. Sounds like a rod. Ideas?
    #1
  2. Big Bamboo

    Big Bamboo Aircooled & Sunbaked

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    Could it be pinging (sounds like rocks rattling in a can) ...or is it a steady knock knock sound? If so, retard the timing a bit and see if that helps.
    #2
  3. Plaka

    Plaka Brevis illi vita est

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    Change the oil. Put in strait 30 or 40wt. and see what happens. If it shuts up or it has to get a whole lot hotter before it starts I would suspect a rod. You can also put a pressure gauge on it (pull the idiot light sender, it's m12x1 thread IIRC) and see if the knock happens as the oil pressure drops under load.

    Check with a mechanics stethoscope at the cylinder base at idle and reving in the driveway. It may be knocking then but not loud enough to get through years of hard rock and power tools hearing damage.

    Also maybe drain the tank into your truck and run some of the highest test gas you can find in the motor. That is diagnostic for detonation type knocks. Or add a can of octane booster to what is in there.
    #3
  4. ME 109

    ME 109 Long timer

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    Sounds like a rod bearing.

    I'd pull the oil filter and dissect it for more clues.
    #4
  5. Cogswell

    Cogswell Spudly Adventurer

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    Pull it apart for inspection before any further damage is done ?


    Mike
    #5
  6. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    I'll third the notion of a rod bearing. Quick check would be to read the oil filter. Is the maintenance/history of the bike known?

    --Bill
    #6
  7. Kai Ju

    Kai Ju Long timer

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    What does it sound like in gear, stationary and starting to let the clutch out while you hold the rear brake on ?
    If it knocks under those conditions it will likely be a rod versus detonation.
    You know well enough how easy it is to verify and if I remember correctly it's usually the right side that let's go first.
    If you catch it early enough you can clean the rod journal up with crocus cloth and just replace the bearing inserts.

    Do you have any history on this engine ?
    #7
  8. supershaft

    supershaft because I can

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    Inspect the filter? You might find the problem before you pull the filter out. You know? A $2000 O-ring. BTDT a number of times.
    #8
  9. ME 109

    ME 109 Long timer

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    Quite possibly so. Or not.
    The filter may reveal dark secrets, or not.

    We want to look in the filter for colours for big end wear, silver and copper iirc?
    #9
  10. pommie john

    pommie john Long timer

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    My money's on a rod bearing being marginal. With cold oil there may be enough oil pressure to fill the gap but it gets noisy when the oil heats up.
    As for going away at high RPM, again maybe enough oil pressure or maybe the knock "blurs" into a continuous noise that can't be distinguished as a knock.
    #10
  11. Stan_R80/7

    Stan_R80/7 Beastly Gnarly

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    I have never had a rod knock under acceleration. If the knock happens under light load (i.e. constant speed), or it knocks after the engine warms up when stationary and letting the clutch out with the rear brake engaged - I would pull the cylinders and inspect the rod crank journals. The most daunting part of pulling the cylinders is removing the exhaust if it has not been off in a good while. Inspecting the journals is the only way to know for sure. Good luck!
    #11
  12. Rucksta

    Rucksta SS Blowhard

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    Can the big ends be accessed by dropping the sump?
    #12
  13. ME 109

    ME 109 Long timer

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    Absolutely.

    Have you got a 9" grinder?
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  14. Rucksta

    Rucksta SS Blowhard

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    Absolutely.

    If Santa brings me a plasma cutter the grinder will become my third favourite tool.
    5lb sledge with the 4 foot handle is the number one tool in my shed.
    #14
  15. ME 109

    ME 109 Long timer

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    Plasma is so...........Star Wars man.

    We got some nice abrasive water streams.
    Or a 5lb hammer. :D
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  16. Airhead Wrangler

    Airhead Wrangler Long timer

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    Maybe try checking oil pressure when it's fully warmed up. If something has a loose enough fit to knock, then I bet it'll be bleeding a fair amount of oil pressure by the time the oil gets hot and runny.
    #16
  17. Jay.T

    Jay.T Adventurer

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    Same issue when I installed copper "compression" gaskets. When I would pin the throttle it would ping. It was more noticeable if rpms were low and at least one other condition- bike was loaded with gear, or headed up this hill by my house. If I kept the rpms up, the is wasn't a problem, or I couldn't hear it.
    #17
  18. Plaka

    Plaka Brevis illi vita est

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    No. You got the cam and the bores for the cam followers in the way. And then the rod bolts face sideways, not down. You wanna get at the conrod big ends you have to go in through the cylinder bore cutouts in the block.
    #18
  19. Plaka

    Plaka Brevis illi vita est

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    You do not need to pull the exhaust to pull the heads.

    It's worth trying the exhaust nuts with the heads on the bike, and use heat. But if cutting the nuts is in order it's easier to pull the head and do the operation on the bench. Cut the nuts in several places, don't go all the way through so you never touch the threads in the heads and then fracture them.
    #19
  20. Rob Farmer

    Rob Farmer Long timer

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    I had the same symptoms with this one. Ran really sweet, chattered when changing gear and knocked at low rpm.

    [​IMG]

    Crank was still good though

    [​IMG]
    #20